Power generation systems often include a power converter that is configured to convert an input power into a suitable power for application to a load, such as a generator, motor, electrical grid, or other suitable load. For instance, a power generation system, such as a wind turbine system, may include a power converter for converting variable frequency alternating current power generated at the generator into alternating current power at a grid frequency (e.g. 50 Hz or 60 Hz) for application to a utility grid. An exemplary power generation system may generate AC power using a wind-driven doubly fed induction generator (DFIG). A power converter can regulate the flow of electrical power between the DFIG and the grid.
In general, the output parameters of a DFIG generator typically vary as its rotor speed is adjusted across the generator's operating speed range. For example, FIG. 1 illustrates a graphical representation of the relationship between rotor voltage and rotor speed for a typical 60 Hz DFIG (e.g., a DFIG having a turns ratio of 3, a synchronous rotor speed of 1200 RPM and an operating speed range from 800 RPM to 1600 RPM). As shown, at the extremes of its operating speed range, the DFIG has a rotor slip of +/−0.33, a rotor frequency of +/−20 Hz. In addition, the rotor emf magnitude is generally equal to the stator emf magnitude. However, as the rotor speed is increased or decreased towards the synchronous speed, such output parameters generally approach zero. For example, as shown in FIG. 1, the rotor frequency crosses through DC at the synchronous speed.
Additionally, FIG. 2 illustrates a graphical representation of the relationship between power and rotor speed for the same 60 Hz DFIG (assuming that a constant power is delivered from the DFIG's stator). The total power (line 202) flowing from the DFIG to the grid may be expressed as the summation of the stator power (line 204) and the rotor power (line 206), with the rotor power 206 being a function of the rotor speed. As shown in FIG. 2, for rotor speeds above the synchronous speed (i.e., super-synchronous speeds), the rotor power 206 is positive and flows from the rotor into the grid. In contrast, for rotor speeds below the synchronous speed (i.e., sub-synchronous speeds), the rotor power 206 is negative and flows from the grid into the rotor. However, when the rotor speed is equal to the synchronous speed (e.g., 1200 RPM), the rotor power 206 is equal to zero.
At or near the synchronous speed of a DFIG system, conventional power converters typically operate at relatively constant current, and consequently the average power loss in an IGBT remains relatively constant. However, as shown in FIG. 1, as the generator speed approaches the synchronous speed, the rotor fundamental frequency approaches DC. Because the transient thermal resistance of the IGBT increases at low frequency, the peak temperature of the rotor side IGBT increases at or near the synchronous speed, resulting in a reduction of the total output current capability of the rotor side of the converter. In addition, operation of a DFIG generator at or near synchronous speed is even more complicated because current harmonics feed through the generator from the rotor side to the stator side and then directly to the transmission utility grid. These harmonics must be controlled to levels dictated by utility grid harmonic requirements. As the speed of the generator approaches the synchronous speed of a DFIG system, the thermal cycling of the IGBT junction increases, again based on the transient thermal resistance of the IGBT, which leads to the switching elements wearing out prematurely.
Accordingly, a system and method that operates a power converter in a way to reduce the power loss of the convertor's switching elements when a generator is operating at or near its synchronous speed would be welcomed in the technology. Ideally the power loss reduction at or near the synchronous speed of a DFIG system would allow a converter to operate without reducing the total output current capability of the rotor side of the converter.